<!DOCTYPE html>
<html lang="zh-CN">

  <head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
  </head>

  <body>
    <button id="btn">开关</button>
    <script>
      let btn = document.getElementById('btn');
      let body = document.body;
      let flag = true;
      btn.onclick = function () {
        body.style.backgroundColor = flag ? "black" : "#ffffff";
        flag = !flag;
      }
    </script>
  </body>

</html>